The environmental job categories in the United States with the highest median pay rates include environmental engineers (2010 median: $78,740) and hydrologists (2010 median: $75,690). In almost all cases these require a master's or advanced degree; fortunately, the educational investment is likely to pay off because openings will be plentiful. The U.S. Bureau of Labor Statistics projected jobs for environmental engineers will increase by 22 percent from 2010 to 2020, while the jobs for hydrologists will increase by 18 percent.

By contrast, the projected growth for all job categories in the U.S. economy during the same period is 14 percent, and the median pay for all occupations in 2010 was $33,840, according to BLS.

BLS also predicts better-than-average projected growth of 18 percent for environmental scientists and specialists, a category with 89,400 existing U.S. jobs in 2010, according to the agency. Median pay for these professionals in 2010 was $61,700.

A master's degree is required for hydrologists, who collect water samples, test for pollution levels, and solve problems of water quality and availability. While some environmental employment categories are available to candidates with a bachelor's degree, a master's or advanced degree is a big advantage. As EPA's Frequently Asked Questions online page about its hiring process explains:

Q: Do graduate degrees play an important role in advancement opportunities at EPA? If so, which specific degrees are you seeking?

A: For many scientific positions, we actively seek people with advanced degrees. Advanced degrees can be used in lieu of experience. For example, having a Master's Degree could qualify you at the GS-9 level and having a Ph.D. Degree could qualify you at the GS-11 level. Many of the scientific positions, such as biologist or chemist, have a specific education requirement like a degree in that discipline. Once on board, there is no general, agency-wide requirement for an advanced degree to enhance promotional opportunity, but it can be a factor depending on the needs of the particular part of EPA.

Take the example of Lisa Jackson, EPA administrator during President Obama's first term. Jackson, a summa cum laude graduate of Tulane University, has a master's degree in chemical engineering from Princeton University. She joined EPA as a staff-level scientist in 1987 and worked in EPA's Region 2 office in New York City. She moved to the New Jersey Department of Environmental Protection in 2003 and was appointed commissioner of that agency in 2006.

Environmental compliance officers also are in demand. Government agencies at all levels and oil and gas companies are top employers for these positions. Nonprofit groups are instrumental in the worldwide environmental industry, and advanced degrees are key for those seeking their higher positions. Almost all of the executive team members of the World Wildlife Fund have them, for instance.